Recently, a water absorbing agent having a high water absorbency has been developed and variously used for mainly disposable usage as absorbing goods such as paper diapers, sanitary napkins, and the like, moreover, as water retaining agents for agriculture/horticulture uses, as water-stop materials for industrial uses, and the like. Many monomers and hydrophilic polymers have been proposed as a material of such water absorbing agent. Especially, an absorbing resin of a polyacrylic acid and/or a salt thereof, for which a polyacrylic acid and/or a salt thereof as a monomer is used, has been most used for industrial purposes due to its high water-absorbing property.
Conventionally, as a water-absorbing property which is desirable for the water absorbing agent, there has been known various properties (parameters) such as absorbency against no pressure, absorbency against pressure, absorption speed, liquid permeability against no pressure, liquid permeability against pressure, impact resistance, tolerance against urine, fluidity, gel strength, grain size, and the like. Moreover, even in the same property (for example, absorbency under no load), a variety of definitions (parameter measurement technique) has been proposed from various viewpoints.
These water absorbencies are mainly used for sanitary/hygienic materials such as paper diapers, sanitary napkins, and the like. Therefore, it is required that the water absorbing agent be white before carried from a factory, in order to give sensation of cleanness and in order not to give uncomfortable sensations because of coloring when the powdery water absorbing agent is compounded with white pulps in the sanitary/hygienic material. In addition, the water absorbing agent is generally white powder. It has been known that even after shipped from a factory, the water absorbing agent is colored (from yellow to brown) as time passes by while being stored or transported, or moreover, when used for the sanitary/hygienic materials. Therefore, it is required that the water absorbing agent be white in the case where absorbing goods are stored over a prolonged time. In recent years, content ratio (percentage by weight) of the absorbing resin in the sanitary/hygienic material tends to increase. Accordingly, the color problem becomes more important.
In order to improve such problem, the following methods are suggested: a method in which an alkyl phosphate or a salt thereof is subsequently added to the absorbing resin (Patent Citation 1, 2); a method in which a total amount of a hydroquinone and benzoquinone in an acrylic acid is prepared not to be more than 0.2 ppm by weight (Patent Citation 3); a method in which a methoxyphenol compound in an acrylic acid is prepared in a range of 10 to 160 ppm by weight (Patent Citation 4); a method in which an inorganic reducing agent is added to the absorbing resin (Patent Citation 5); a method in which an organic carboxylic acid or a salt thereof is added to the absorbing resin (Patent Citation 6, 7, 8); a method in which a tocopherol (Patent Citation 9) or a sterically-hindered phenol (Patent Citation 10) is used as a polymerization inhibitor in an acrylic acid for polymerization; a method in which a metal chelating agent is added in producing an absorbing resin (Patent Citation 11, 12); and a method in which an acrylic acid monomer and/or a salt thereof is polymerized by a hydroxyperoxide and a reducing agent, and then processed by a silane coupling agent (Patent Citation 13). However, in any of the above methods, the color problem after the absorbing resin is produced is not sufficiently improved, and moreover, there still have been problems such as polymerization reactor control in producing the absorbing resin, property deterioration, and the like.
Conventionally, a methoxyphenol used in treating of unsaturated-monomers such as an acrylic acid and the like is generally used in a range of about 100 to 200 ppm by weight from the viewpoint of stability of storage and preservation. However, it is difficult to improve coloring status because of increase in the surface color of the absorbing resin due to the methoxyphenol. Meanwhile, in order to improve the surface color of the absorbing resin, a technique to control the methoxyphenol in a certain amount range has been proposed. However, when the absorbing resin is produced from the unsaturated-monomer through polymerization step, drying step, and the like steps at actual production facilities, the technique is insufficient to produce desired properties.
Especially, in order to remarkably improve the surface color of the absorbing resin, when the methoxyphenol is used, for example, by 20 ppm by weight at an actual production facility, it is extremely difficult to produce an absorbing resin having stability and high properties.
Moreover, in conventionally proposed methods using an additive agent or the like, when the whiteness of the surface color of particles becomes better, the polymerization control becomes difficult, and it causes problems such as increase in residual monomers and soluble contents, or the like.
More specifically, in the conventional methods, when absorbency is improved and an amount of the residual monomers is decreased, an amount of the soluble contents increases and the surface color of the particle-shaped water absorbing agent is colored. When the absorbency is improved and the amount of the soluble contents is decreased, the surface color is improved to some extent, but the improvement is not sufficient, and furthermore, the amount of the residual monomers increases.
As described above, it is difficult to satisfy either of the water-absorbing property (the absorbency, the amount of the soluble contents, and the amount of the residual monomers) and the surface color of the particle-shaped water absorbing agent (especially, yellow which is represented of b value) by the conventional methods such as radical control, addition of an oxidizing agent or reducing agent, control of an amount of impurity, or the like. In addition, a conventional method of adding a chelating agent, for example, has an advantage of preventing coloring for storage over time or under high temperature and humidity. However, because the additive agent has its own color, it is not sufficient to satisfy either of the drastic improvement of the surface color of the particle-shaped water absorbing agent and the water-absorbing property.
When the ratio of the absorbing resin and the particle-shaped water absorbing agent in an absorber is as low as 10 to 30% by weight, those problems are not obvious. However, in recent years, as a density of the absorbing resin becomes high (the ratio thereof in the absorber or the paper diapers is 40 to 100% by weight), those problems has come to the surface.    [Patent Citation 1] Japanese Unexamined Patent Publication, Tokukaihei, No. 5-86251 (published on Apr. 6, 1993)    [Patent Citation 2] Japanese Unexamined Patent Publication, Tokukaihei, No. 1-275661 (published on Nov. 6, 1989)    [Patent Citation 3] U.S. Pat. No. 6,444,744 (registered on Sep. 3, 2002)    [Patent Citation 4] International Publication No. 2003/51940 (published on Jun. 26, 2003)    [Patent Citation 5] International Publication No. 2000/55245 (published on Sep. 21, 2000)    [Patent Citation 6] Japanese Unexamined Patent Publication, Tokukai, No. 2000-327926 (published on Nov. 28, 2000)    [Patent Citation 7] Japanese Unexamined Patent Publication, Tokukai, No. 2003-52742 (published on Feb. 25, 2003)    [Patent Citation 8] Japanese Unexamined Patent Publication, Tokukai, No. 2005-186016 (published on Jul. 14, 2005)    [Patent Citation 9] International Publication No. 2003/53482 (published on Jul. 3, 2003)    [Patent Citation 10] International Publication No. 2005/54356 (published on Jun. 16, 2005)    [Patent Citation 11] Japanese Unexamined Patent Publication, Tokukai, No. 2003-206305 (published on Jul. 22, 2003)    [Patent Citation 12] Japanese Unexamined Patent Publication, Tokukai, No. 2003-206381 (published on Jul. 22, 2003)    [Patent Citation 13] Japanese Unexamined Patent Publication, Tokukaihei, No. 4-331205 (published on Nov. 19, 1992)